1. Field of the Invention
The present invention relates to an optical fiber and an optical fiber preform.
2. Description of the Background Art
A silica glass optical fiber which contains an alkali metal element in the core region is described in JP2005-537210A, US2006/0130530A, JP2007-504080A, JP2008-536190A, JP2010-501894A, JP2009-541796A, JP2010-526749A, WO98/002389, and U.S. Pat. No. 5,146,534. It is said that if the core portion of an optical fiber preform contains an alkali metal element, the viscosity of the core portion can be decreased and the relaxation of the network structure of silica glass is enhanced when the optical fiber preform is drawn into a fiber, and consequently the attenuation of a resultant optical fiber can be reduced.
JP2005-537210A and US 2006/0130530A describe a diffusion method as a method of adding an alkali metal element to silica glass. The diffusion method is a method for carrying out diffusion doping of an alkali metal element into the inner surface of a glass pipe by heating the glass pipe by means of an outside heat source, or by generating plasma inside the glass pipe, while introducing materials vapor containing the alkali metal element or an alkali metal salt into the glass pipe.
After the glass pipe is doped with an alkali metal element in the vicinity of inner surface thereof in the above-mentioned manner, the glass pipe is heated to reduce its diameter. After such diameter reduction, the inner surface of the glass pipe is etched by a given thickness to remove contamination including water and transition metal elements such as Ni and Fe which have inevitably been added simultaneously when the alkali metal element are added. Since the diffusion of the alkali metal element is faster than that of the transition metal elements, it is possible to cause the alkali metal element to remain even if the transition metal elements are removed by etching a given thickness of the glass surface. After such etching, the glass pipe is collapsed by heating, and thereby a core rod containing the alkali metal element is prepared. A cladding is formed on the outside of the core rod containing alkali metals, whereby an optical fiber preform is produced. An optical fiber can be manufactured by drawing the optical fiber preform into a fiber.